Photochromic compounds based on ring opening and closing of an [1,3]oxazine compound

ABSTRACT

We have designed a molecular switch based on the photoinduced opening and thermal closing of a [1,3]oxazine ring. A substituted [1,3]oxazine compound described as having a general (i.e., unsubstituted) structure with fused indoline and benzooxazine fragments such that they share a common bond in the [1,3]oxazine compound: (i) the bond connecting positions 1 and 2 of the indoline fragment and (ii) the bond connecting positions 2 and 3 of the benzooxazine fragment. Irradiation by light of suitable wavelength and intensity of this photochromic compound induces cleavage of a [C—O] bond of the [1,3]oxazine ring to form a phenolate chromophore. The photogenerated (e.g., colored) isomer may revert thermally to the starting (e.g., colorless) oxazine. Alternatively, the switch may be between isomers of the compound that absorb at different wavelengths. Reversible coloration of silica or polymeric materials and switching optical signals may involve many cycles of interconversion between different colored states. A colorless/colored state may be maintained by constant irradiation or chemical trapping.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is application is a Divisional of U.S. application Ser. No.11/708,132, filed Feb. 20, 2007, now U.S. Pat. No. 7,790,068 whichclaims priority benefit of provisional U.S. Application No. 60/774,190,filed Feb. 17, 2006; the entire contents of which are herebyincorporated by reference.

FEDERALLY-SPONSORED RESEARCH OR DEVELOPMENT

The U.S. Government has certain rights in this invention as provided forby the terms of CHE-0237578 awarded by the National Science Foundation.

BACKGROUND OF THE INVENTION

This invention relates to substituted [1,3]oxazine compounds with ageneral (i.e., unsubstituted) structural formula described by fusedindoline and benzooxazine fragments such that they are fused along thebond connecting positions 1 and 2 of the indoline fragment and the bondconnecting positions 2 and 3 of the benzooxazine fragment. Theseproducts, as well as processes to make them or to use them, areprovided.

Photochromic compounds change their color when illuminated. In mostcases, a colorless compound switches to a colored compound uponillumination by light of suitable wavelength and intensity. Thephotogenerated species reverts to the starting species by either thermalmeans or further illumination. These reversible chemical transformationsare accompanied by pronounced structural and electronic modifications,which often alter the ability of the photochromic compound to emitlight. Under these conditions, the photoinduced and reversibleinterconversion of the colorless and colored states results in themodulation of the fluorescence intensity. Thus, this mechanism oflight-induced transformation and reversion can be exploited to regulatethe emissive behavior of collections of molecules in solution and evenin solids. The investigation of these fascinating systems have led tonew light-responsive materials for applications such as ophthalmiclenses for corrective or cosmetic purposes, optical filters toselectively transmit light, optical limiters to nonlinearly decreasetheir transmittance in response to increased incident light, photonicswitches (including routers) to enable optical communication, public orpersonal displays of text or pictures, and silica or polymeric panesadapted for installation in “smart” windows of homes and buildings.

The term “photochromism” indicates a photoinduced change in color.Rather than the interconversion between two colored states, however,these transformations usually involve a transition from a colorlessstate to a colored state. Though less common, photoinduced transitionsfrom colored to colorless forms are also possible. Indeed, thedefinitions “positive photochromism” and “negative photochromism” areoften employed to distinguish coloration and decoloration processes,respectively. In any case, a photochromic transformation is alwaysaccompanied by, profound absorbance changes in the visible region. Infact, visible absorption spectroscopy is the most convenient analyticalmethod to study these processes.

Reversibility is an essential requirement for photochromictransformations. The photoinduced absorbance changes must be reversibleby definition. In fact, photochromic compounds can be classified intotwo broad categories depending on the nature of the reverse process.Both classes share in common the ability to switch from one state toanother when irradiated by light. Thermally stable photochromiccompounds retain the photogenerated state even after turning off thelight source, but return to the starting state after irradiation at adifferent wavelength. Thermally reversible photochromic compounds,instead, return to the starting state spontaneously when the irradiationis terminated.

Photochromic transformations are generally based on either unimolecularor bimolecular reactions. In most instances, unimolecular photochromicprocesses involve interconversion between two isomer forms. They can bebased on light-induced ring opening/closing, cis/trans isomerizations,or intramolecular proton transfer. Bimolecular photochromic processesare less common. They rely on either the photoinduced cycloaddition oftwo identical reactants into a single product or on the photoinducedtransfer of an electron from a donor to a complementary acceptor.

We have investigated a spiropyran as a photochromic compound, but itsuffers at least two major limitations. Its thermal re-isomerization isrelatively slow. Thus, restoration of the starting state is delayed byseveral minutes once the light is turned off and many applicationsrequire a quicker response. Furthermore, our spiropyran tolerates only alimited number of switching cycles.

Therefore, it is an objective of the invention to provide an improvedclass of photochromic compounds that undergo photoinducible [1,3]oxazinering opening and revert by [1,3]oxazine ring closing. Irradiation of acompound triggers photo-induced cleavage of a [C—O] bond of the[1,3]oxazine ring to form a phenolate chromophore. The photogenerated(colored) isomer may revert thermally to the starting (colorless)isomer. Reversible coloration of transparent or translucent silica orpolymeric materials and light-induced switching may involve multiplecycles of interconversion between different colored states. Ourinvention addresses the need for improved an photochromic compound with:(i) faster switching speed and (ii) better fatigue resistance thanspiropyran compounds.

The present invention is directed to improved optical materials andsystems for photochromic switching between two different optical states(e.g., a difference in light absorbance). Other advantages andimprovements are described below or would be apparent from thedisclosure herein.

SUMMARY OF THE INVENTION

An objective is providing a substituted [1,3]oxazine compound, whereinthe unsubstituted [1,3]oxazine compound is described as having astructure with fused indoline and benzooxazine fragments such that theyshare a common bond in the [1,3]oxazine compound: (i) the bondconnecting positions 1 and 2 of the indoline fragment and (ii) the bondconnecting positions 2 and 3 of the benzooxazine fragment. Other thanoptional substitutions at one or more positions on the fused ringsystems, there is a further substitution of an electron withdrawinggroup attached at any position of a phenyl ring of the system. It ispreferred that the carbon atom between nitrogen and oxygen of the[1,3]oxazine ring be a tertiary carbon that is the position of one ofthe optional substitutions.

In one embodiment, the substituted [1,3]oxazine compound has Formula I.Selection of R¹, R⁴, and R⁵ can determine (i) the compound'sresponsiveness to light (e.g., the excitation wavelength and intensitythat induces ring cleavage) and (ii) the absorbance wavelength of thephenolate derivative. Selection of R² and R³ can affect the rates ofisomerization and/or re-isomerization. In particular, colorless andcolored states may be switched by irradiating the compound with visiblelight of suitable wavelength and intensity, and then removing the lightsource to allow thermal re-isomerization.

R¹ may be hydrogen, hydroxyl, C1-C4 alkyl, C5-C6 cycloalkyl, substitutedC1-C4 alkyl, substituted C5-C6 cycloalkyl, C5-C6 aryl, substituted C5-C6aryl, C5-C6 heterocycle, or substituted C5-C6 heterocycle. R¹ may bepositioned at any position on a phenyl of the fused ring system, but theposition opposite the attachment point of the nitrogen atom ispreferred. R² and R³ may be the same or different: hydrogen, hydroxyl,C1-C4 alkyl, C5-C6 cycloalkyl, substituted C1-C4 alkyl, substitutedC5-C6 cycloalkyl, C5-C6 aryl, substituted C5-C6 aryl, C5-C6 heterocycle,or substituted C5-C6 heterocycle. R⁴ may be hydrogen, hydroxyl, C1-C4alkyl, C5-C6 cycloalkyl, substituted C1-C4 alkyl, substituted C5-C6cycloalkyl, C5-C6 aryl, substituted C5-C6 aryl, C5-C6 heterocycle, orsubstituted C5-C6 heterocycle. R⁵ may be a nitrogen-containing group orany other electron withdrawing substituent (e.g., cyanide and halidessuch as chloro, bromo, and fluoro); it may be positioned at any positionon a phenyl of the fused ring system, but the position opposite theattachment point of the oxygen atom is preferred.

Also provided are processes for using and making the products. Forexample, [1,3]oxazine compounds may be synthesized by fusing indolineand benzooxazine fragments (e.g., N-alkylating2-R⁴-3,3′-R²,R³-6-R¹-3H-indole with 2-chloromethyl-4-R⁵-phenol toproduce an intermediate and then cyclizing the intermediate under basicconditions). Using such compounds by irradiating them with light (e.g.,from 200 nm to 800 nm, 800 nm to 1300 nm, etc.) opens the [1,3]oxazinering by cleaving a [C—O] bond to generate a phenolate chromophore thatis able to absorb light, preferably visible. Reversion occurs byreformation of the [C—O] bond. The hemiaminal form may be trapped byusing a nucleophile. Optical filters, optical limiters, ophthalmiclenses, photonic switches, or window panes that respond rapidly to achange in light and maintain this property through many cycles may befabricated.

Further aspects of the invention will be apparent to a person skilled inthe art from the following description of specific embodiments and theclaims, and generalizations thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of the reversible interconversion betweencolorless spiropyran (SP) and colored merocyanine (ME) isomers.

FIG. 2 is a schematic of the synthesis of [1,3]oxazine compounds OX1 andOX2.

FIG. 3 shows (FIG. 3A) the compound OX1 and (FIG. 3B) its partial ¹H-NMRspectra (500 MHz, CD₃CN, 5 mM) at 2° C. (a), 10° C. (b), 20° C. (c), 40°C. (d), 50° C. (e), and 63° C. (t).

FIG. 4 shows (FIG. 4A) the compound OX1 and (FIG. 4B) its partial ¹H-NMRspectra (400 MHz, CD₃CN, 10 mM) before (a) and after (b) the addition ofBu₄NOH (2 equiv.).

FIG. 5 shows (FIG. 5A) the compound OX2 and (FIG. 5B) its partial ¹H-NMRspectra (500 MHz, CD₃CN, 5 mM) at 30° C. (a), 60° C. (b), and 70° C.(c).

FIG. 6 shows (FIG. 6A) the compound OX2 and (FIG. 6B) its partial ¹H-NMRspectra (400 MHz, CD₃CN, 10 mM) before (a) and after (b) the addition ofBu₄NOH (2 equiv.).

FIG. 7 shows steady-state absorption spectra (0.1 mM, MeCN, 25° C.) of(FIG. 7A) OX1 (a), 5 (b) and 7 (c) as well as (FIG. 7B) OX1 after theaddition of Bu₄NOH (1 equiv.) (d) and 8 (e).

FIG. 8 shows steady-state absorption spectra (0.1 mM, MeCN, 25° C.) of(FIG. 8A) OX2 (a), 6 (b) and 7 (c) as well as (FIG. 8B) OX2 after theaddition of Bu₄NOH (100 equiv.) (d) and 8 (e).

FIG. 9A shows a transient absorption spectrum of (a) OX1 recorded 30 nsafter the laser pulse (355 nm, 6 ns, 8 mJ, 0.1 mM, MeCN, 22° C.) and asteady-state absorption spectrum (b) OX1 and 1 equiv. Bu₄NOH (0.1 mM,MeCN, 25° C.). FIG. 9B shows the evolution of the absorbance at 440 nm(c) upon laser excitation of OX1 and the corresponding mono-exponentialcurve fitting (d).

FIG. 10A shows a transient absorption spectrum (a) of OX2 recorded 30 nsafter the laser pulse (355 nm, 6 ns, 8 mJ, 0.1 mM, MeCN, 22° C.) and asteady-state absorption spectrum (b) of OX2 and 100 equiv. Bu₄NOH (0.1mM, MeCN, 25° C.). FIG. 10B shows the evolution of the absorbance at 440nm (c) upon laser excitation of OX2 and the correspondingmono-exponential curve fitting (d).

FIG. 11A shows steady-state absorption spectra (0.1 mM, MeCN, 20° C.) ofOX2 (a), p-nitroanisole (b), potassium p-nitrophenolate (c), OX2 afterthe addition of Bu₄NOH (10 equiv.) and continuous irradiation (365 nm,400 μW cm⁻²) for 10 min (d). FIG. 11B shows the transient absorptionspectrum (0.1 mM, MeCN, 22° C.) of OX2 (e) recorded 30 ns after a laserpulse (355 nm, 6 ns, 8 mJ).

FIG. 12A shows steady-state absorption spectra (5%, PMMA, 25° C.) of OX1without (a) and with (b) Bu₄NOH (7 equiv.) and transient absorptionspectra of OX1 recorded 1 μs (c) after the laser pulse (355 nm, 6 ns, 12mJ, 5%, PMMA, 22° C.). FIG. 12B shows the evolution of the absorbance at380 nm (d) upon laser excitation of OX1 (355 nm, 6 ns, 12 mJ, 5%, PMMA,22° C.) and the corresponding mono-exponential curve fitting (e).

FIG. 13A shows steady-state absorption spectra (5%, PMMA, 25° C.) of OX2without (a) and with (b) Bu₄NOH (7 equiv.) and transient absorptionspectra of OX2 recorded 1 μs (c) or 80 μs (d) after the laser pulse (355nm, 6 ns, 12 mJ, 5%, PMMA, 22° C.). FIG. 13B shows the evolution of theabsorbance at 380 nm (e) upon laser excitation of OX2 (355 nm, 6 ns, 12mJ, 5%, PMMA, 22° C.) and the corresponding bi-exponential curve fitting(f).

FIG. 14 shows the absorbance at 380 nm upon laser excitation of OX2 (355nm, 6 ns, 12 mJ, 5%, PMMA, 22° C.) before (a) and after (b) 1000excitation cycles.

FIG. 15 shows evolution of the absorbance at 430 nm (a) upon laserexcitation of OX2 (355 nm, 6 ns, 12 mJ, 5%, PMMA, 22° C.) and thecorresponding bi-exponential curve fitting (b).

FIG. 16 shows the absorbance of indolium at 440 nm during threeconsecutive switching cycles of OX2 (0.1 mM, MeCN, 22° C.) performed bylaser excitation (355 nm, 6 ns, 8 mJ).

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

The structural and electronic changes that accompany the light-inducedtransformation of photochromic compounds have suggested a wealth ofapplications over the past four decades. These compounds have emerged asconvenient building blocks for constructing photoswitchable assemblies.In these systems, interconversion between the different states of aphotochromic compound may modulate the optical properties of the overallassembly. The promise offered by photonic technology may be advanced bythe novel photochromic compounds of the invention with the goal ofdeveloping innovative light-responsive materials for display,information processing, storage, therapy, and visualization.

Photochromic compounds change their structural and electronic propertiesin response to optical stimulation. The photogenerated state reverts tothe starting state either thermally or after the application of a secondoptical stimulation differing in wavelength from the first.Thermally-reversible photochromes offer the opportunity to alter andreset the state of an output property (e.g., absorption or refractiveindex) by simply turning on and off a light source, since they do notretain the influence of the incident radiation. The ability to restoreoutput level and the lack of memory effects are essential conditions forthe implementation of combinational logic functions. As a result, thesecompounds have emerged as possible building blocks for the constructionof molecular logic gates. Indeed, recent investigations havedemonstrated that collections of photochromic compounds in solution orwithin rigid matrices can reproduce logic functions relying on theinterplay of optical signals.

Photochromic compounds are the building blocks of light-responsivematerials (e.g., ophthalmic lenses for corrective or cosmetic purposes,optical filters or limiters to modulate light signals, photonic switchesto enable optical communication, public or personal displays of text orpictures, and silica or polymeric panes adapted for installation in“smart” windows of homes, offices, and vehicles). They generally requireseveral minutes to switch from a colorless to a colored state and viceversa. These relatively slow processes limit the switching times ofcurrent photochromic materials, which need several minutes to adjust inresponse to changes in light intensity. Our compounds, instead, canswitch between colorless and colored states in nanoseconds. Their rapidisomerization kinetics can, therefore, translate into an improvement often orders of magnitude in the switching times of photochromicmaterials.

Light-responsive public displays (e.g., billboards and stadiumscoreboards) and personal displays (e.g., mounted on vertical orhorizontal surfaces), filters and/or limiters in optical assemblies andsystems, photonic switches for optical communication, ophthalmic lensesof contacts and glasses, and “smart” window panes adapted forinstallation in a home, office, or vehicle window may be fabricatedusing these photochromic compounds in optical materials. For example,the optical material may darken (i.e., absorb light) in response toirradiation by light of suitable wavelength and intensity.

The photoinduced isomerization of spiropyrans (e.g., SP in FIG. 1)involves two consecutive steps. Upon ultraviolet irradiation, the [C—O]bond at the spirocenter cleaves in picoseconds. Then, the adjacent [C═C]bond switches from a cis to a trans configuration in microseconds. Thethermal transformation of the resulting merocyanine (e.g., ME in FIG. 1)back to the starting spiropyran is slowed significantly by a necessarytrans→cis re-isomerization step. For example, ME switches back to SP inminutes with a rate constant of about 25×10⁻⁴ s⁻¹ in MeCN at 25° C.

The present invention provides a substituted [1,3]oxazine compound,wherein the unsubstituted [1,3]oxazine compound is described as having astructure with fused indoline and benzooxazine fragments such that theyshare a common bond in the [1,3]oxazine compound: (i) the bondconnecting positions 1 and 2 of the indoline fragment and (ii) the bondconnecting positions 2 and 3 of the benzooxazine fragment. The two fusedheterocycle fragments are constrained with respect to each other suchthat the dihedral angle formed between the axis of the 2p_(z) orbital onthe indoline nitrogen atom and the adjacent σ_(C—O) orbital is acute(e.g., 15° to 30°). Bulky substituents near the dihedral angle areavoided. Although substitutions at one or more positions of the fusedring systems (e.g., preferred are one, two, three, or four substitutionsfor a hydrogen of the unsubstituted [1,3]oxazine compound) may beoptional, substitution of an electron-withdrawing substituent at anyposition of a phenyl ring of the fused ring system is mandatory.

It is preferred that one of the optional substitutions occurs at thecarbon atom between nitrogen atom and oxygen atom of the [1,3]oxazinering to provide a tertiary carbon. In such embodiments, this chiralcarbon atom and the nitrogen atom are shared between the two fusedheterocycle fragments of the [1,3]oxazine compound.

In one embodiment, the substituted [1,3]oxazine compound has Formula I.Cleavage of a [C—O] bond in the [1,3]oxazine ring produces a phenolatederivative, which preferably absorbs visible light. Selection of R¹, R⁴,and R⁵ can determine (i) the compound's responsiveness to light (e.g.,the excitation wavelength and intensity that induces ring cleavage) and(ii) the absorbance wavelength of the phenolate derivative. Selection ofR² and R³ (e.g., methyl substituents) can affect the rates ofisomerization and/or re-isomerization. In particular, colorless andcolored states may be switched by irradiating the compound with visiblelight of suitable wavelength and intensity, and then removing the lightsource to allow thermal re-isomerization. A hemiaminal form may betrapped with a nucleophile that competes with the ring-closing reactionto form the hemiaminal.

R¹ may be hydrogen, hydroxyl, C1-C4 alkyl (e.g., methyl, ethyl, propyl,butyl) or C5-C6 cycloalkyl, substituted (e.g., halide, hydroxyl) C1-C4alkyl or C5-C6 cycloalkyl, C5-C6 aryl (e.g., furyl, phenyl), substituted(e.g., halide, hydroxyl) C5-C6 aryl (e.g., halide, hydroxyl), C5-C6heterocycle, or substituted (e.g., halide, hydroxyl) C5-C6 heterocycle.R¹ may be positioned at any position on a phenyl of the fused ringsystem, but the position opposite the attachment point of the nitrogenatom is preferred. R² may be hydrogen, hydroxyl, C1-C4 alkyl (e.g.,methyl, ethyl, propyl, butyl) or C5-C6 cycloalkyl, substituted (e.g.,halide, hydroxyl) C1-C4 alkyl or C5-C6 cycloalkyl, C5-C6 aryl (e.g.,furyl, phenyl), substituted (e.g., halide, hydroxyl) C5-C6 aryl (e.g.,halide, hydroxyl), C5-C6 heterocycle, or substituted (e.g., halide,hydroxyl) C5-C6 heterocycle, or fused ring systems (e.g., biphenyl). R³may be hydrogen, hydroxyl, C1-C4 alkyl (e.g., methyl, ethyl, propyl,butyl) or C5-C6 cycloalkyl, substituted (e.g., halide, hydroxyl) C1-C4alkyl or C5-C6 cycloalkyl, C5-C6 aryl (e.g., furyl, phenyl), substituted(e.g., halide, hydroxyl) C5-C6 aryl (e.g., halide, hydroxyl), C5-C6heterocycle, substituted (e.g., halide, hydroxyl) C5-C6 heterocycle, orfused ring systems (e.g., biphenyl). R⁴ may be hydrogen, hydroxyl, C1-C4alkyl (e.g., methyl, ethyl, propyl, butyl) or C5-C6 cycloalkyl,substituted (e.g., halide, hydroxyl) C1-C4 alkyl or C5-C6 cycloalkyl,C5-C6 aryl (e.g., furyl, phenyl), substituted (e.g., halide, hydroxyl)C5-C6 aryl (e.g., halide, hydroxyl), C5-C6 heterocycle, substituted(e.g., halide, hydroxyl) C5-C6 heterocycle, or fused ring systems (e.g.,biphenyl). R⁵ may be a nitrogen-containing group (e.g., nitroso, nitro,azo dyes) or any other electron withdrawing substituent (e.g., cyano,halides). R⁵ may be positioned at any position on a phenyl of the fusedring system, but the position opposite the attachment point of theoxygen atom is preferred. The relative orientation of the fused,substantially planar heterocycles constrains the dihedral angle betweenthe axis of the 2p_(z) orbital on the indoline nitrogen atom and that ofthe adjacent σ_(C—O) orbital. Bulky substituents at R², R³, and R⁴ nearthe dihedral angle are avoided.

The compound may be switched by irradiating with a light source (e.g.,dye or gas laser, lamp, light emitting diode). Switches results in anisomeric shift between the compound and its phenolate derivative, andtheir absorbance of visible light shifts from colorless (startingcompound) to colored (phenolate derivative) by this isomerization.Depending on the excitation wavelength of the compound, the switchresults in the maximum absorbance wavelength to shift by a positive ornegative difference of at least 50 nm, at least 100 nm, at least 150 nm,at least 200 nm, at least 250 nm, or at least 300 nm.

The light may have a wavelength from 200 nm to 1300 nm, 400 nm or more,at least 600 nm or more, at least 800 nm or more, 800 nm or less, 1000nm or less, 1300 nm or less, or any range therebetween. The light sourcemay be a laser diode (e.g., from 200 nm to 800 nm) or a light emittingdiode (LED) (e.g., from 800 nm to 1300 nm). Illumination may be providedby constant or pulsed light. Preferably, the intensity of the light isslow and the time of illumination is short.

A photoinducible optical state may be maintained by constantillumination or by chemical trapping (e.g., nucleophile) of an isomer inthe form of the hemiaminal. Most of the compounds in a composition mayswitch between isomeric states within 5 ns or less, 10 ns or less, 50 nsor less, or 250 ns or less. Compounds remain able to photoactivate andrevert over greater than 1000 excitation cycles, greater than 3000excitation cycles, or greater than 5000 excitation cycles.

The compound may be incorporated in silica, a liquid crystal, or apolymeric material or in one or more sheets of such material as alaminate. The material may be a flexible or rigid solid, preferably itis transparent or translucent. Alternatively, the compound may bedissolved in a liquid (e.g., solution or gel) and then encapsulated in asolid material (e.g., applied in a thin film, cast or molded as a sheet,segregated in beads or laminated structures). The material may beamorphous (e.g., glass) or crystalline (e.g., quartz). Examples ofpolymeric materials include polycarbonate, polymethylmethacrylate, andpolystyrene.

If incorporated in a solid material or encapsulated within a solidmaterial, the material is preferably at least opaque to the wavelengthof light that induces switching in the compound and does not attenuatethe intensity of light such that switching is not efficient. Inparticular, the compounds may be dissolved in an organic solvent and itsfunction is not oxygen sensitive.

Specific embodiments of the invention are the OX1 and OX2 compounds(FIG. 2). In analogy to SP, ultraviolet irradiation of OX1 or OX2induces the cleavage of a [C—O] bond, involving the tertiary carbon ofthe indoline fragment, with the formation of a p-nitrophenolatechromophore. The resulting indolium lacks the central double bond of ME.Thus, the rate of the thermal transformation of indolium back to oxazineshould not be limited by the relatively slow trans→cis re-isomerizationassociated with ME.

We have synthesized OX1 and OX2 in two steps (FIG. 2) with an overallyield of 58%. Specifically, the N-alkylation of either2-methyl-3,3′-dimethyl-3H-indole (1) or 2-phenyl-3,3′-dimethyl-3H-indole(2) with 2-chloromethyl-4-nitrophenol, and then the cyclization underbasic conditions resulted in OX1 or OX2. In general, [1,3]oxazinephotochromic compounds may be synthesized by fusing indoline andbenzooxazine fragments starting from 2-R⁴-3,3′-R²,R³-3H-indoles and2-chloromethyl-4-nitrophenol, wherein R¹ is hydrogen and R⁵ is nitro.For OX1 and OX2, R¹ is hydrogen, R² and R³ are methyl, and R⁵ is nitro.

OX1 and OX2 differ in the substitution attached to the tetrahedralcarbon atom shared by the indoline and benzooxazine fragments. Therelative orientation of these two fused heterocycles constrains thedihedral angle between the axis of the 2p_(z) orbital on the indolinenitrogen atom and that of the adjacent σ_(C—O) orbital to valuescomparable to those (15° to 30°) of nitrospiropyrans. Thus, thisparticular geometry is expected to favor electronic mixing between thetwo orbitals and facilitate cleavage of the [C—O] bond in the excitedstate, as observed for nitrospiropyrans. It follows that ultravioletirradiation of either OX1 or OX2 should open the [1,3]oxazine ring togenerate a 4-nitrophenolate chromophore able to absorb violet light.

The chiral center at the junction of the two fused heterocycles in OX1and OX2 imposes two distinct environments on the pair of indoline methylgroups and the pair of oxazine methylene protons. Consistently, the ¹HNMR spectra of both compounds reveal pairs of singlets for the methylprotons and AB systems for the methylene protons, when recorded inacetonitrile-d3 at ambient temperature. For example, the two singletsfor the methyl protons (Me° and Me^(□)) of OX1 (FIG. 3A) appear at 1.15and 1.51 ppm in the ¹H-NMR spectrum (a in FIG. 3B) recorded at 2° C. TheAB system for the methylene protons (H° and H^(□)) is instead centeredat 4.69 ppm. Upon warming the solution, the two singlets broaden (b to fin FIG. 3B) and eventually coalesce into a single peak. Similarly, thetwo doublets within the AB system broaden and coalesce into a singlepeak. These changes are a result of the interconversion between the twoenantiomers of OX1 on the ¹H NMR time scale. This degenerate siteexchange process demands the thermal cleavage of the [C—O] bond at thejunction of the two heterocycles with the formation of the ring-openedintermediate (FIG. 3A).

The addition of Bu₄NOH to a solution of OX1 (FIG. 4A) has similareffects on the ¹H NMR and mass spectra. Once again, peaks for thecorresponding hemiaminal (FIG. 4B) can be clearly observed. The methylgroup in position 2 of the indolium fragment, however, is relativelyacidic. Thus, the nucleophilic attack of the hydroxide anion to theindolium cation competes with its deprotonation and another compound(FIG. 4A) is formed in parallel to the hemiaminal. Consistently, thebroad resonance for the pair of diastereotopic methylene protons of OX1(a in FIG. 4B) is replaced by an AB system for H^(a) and H^(b) of thehemiaminal and a singlet for H^(a) and H^(b) of the another compound (bin FIG. 4B). In addition, an AB system for the olefinic protons H^(c)and H^(d) of the another compound can also be observed. From theintegrals of these resonances, the ratio between hemiaminal and thecompound is estimated to be 34:66.

The kinetic parameters (Table 1) associated with the ring-openingprocess can be extracted from the analysis of the temperature dependenceof the line widths associated with the singlets for Me° and Me^(□) inthe slow-exchange regime. A similar analysis of OX2 (FIG. 5A) can beextended to the ¹H NMR spectra (FIG. 5B), which reveal essentially thesame behavior.

TABLE 1 Kinetic parameters associated with the thermal ring opening ofOX1 and OX2 at 25° C.^(a) k ΔG^(‡) ΔH^(‡) −ΔS^(‡) Solvent Compound (s⁻¹)(kcal mol⁻¹) (kcal mol⁻¹) (kcal mol⁻¹ K⁻¹) Acetonitrile-d3 OX1 199 ± 7 14.31 ± 0.02 13.5 ± 0.2 0.003 ± 0.001 OX2 0.4 ± 0.1 17.99 ± 0.18 17.4 ±0.3 0.002 ± 0.001 Toluene-d8^(b) OX1 0.081 ± 0.036 19.00 ± 0.29 19.8 ±2.0 −0.003 ± 0.006  ^(a)The rate constant (k), free energy (ΔG^(‡)),enthalpy (ΔH^(‡)) and entropy (ΔS^(‡)) of activation were determined byvariable-temperature ¹H NMR spectroscopy. ^(b)In toluene-d8, the linewidths of the singlets associated with the pair of methyl protons of OX2remain approximately constant in the examined temperature range (2°C.-90° C.). As a result, the kinetic parameters for the ring opening ofthis compound could not be determined.

A comparison of the rate constants (k in Table 1) determined for thethermal ring opening of OX1 and OX2 in acetonitrile-d3 reveals that thegroup on the chiral center at the junction of the two heterocycles has apronounced influence on the kinetics of this process. Indeed, atransition from the methyl group of OX1 to the phenyl ring of OX2translates into a decrease in k from about 199 to 0.4 s⁻¹. This changecorresponds to an increase in free energy barrier (ΔG^(‡) in Table 1) ofabout 3.7 kcal mol⁻¹. Interestingly, the enthalpic term (ΔH^(‡) inTable 1) dominates ΔG^(‡), while the entropic contribution (TΔS^(‡)) at25° C. is less than 1 kcal mol⁻¹ for both compounds. In toluene-d8,TΔS^(‡) remains negligible, but ΔH^(‡) increases by about 4.7 kcal mol⁻¹for OX1. Consistently with the solvent-induced enhancement in ΔH^(‡),the ¹H NMR spectrum of OX1 does not change significantly withtemperature in toluene-d8. Two well-defined singlets for the methylprotons of OX2 can clearly be observed even at 90° C., in agreement withthe slow ring-opening kinetics.

The ring-opened intermediates revert to the corresponding oxazines OX1and OX2 after the intramolecular attack of the 4-nitrophenolate anion tothe adjacent indolium cation (cf. FIGS. 3A and 5A). Nucleophiles able tocompete intermolecularly with the ring-closing step can therefore “trap”these short-lived intermediates. For example, the addition of twoequivalents of Bu₄NOH to a solution of OX2 results in the quantitativeformation of the hemiaminal (FIG. 6A).

In agreement with the formation of the hemiaminal, the ¹H-NMR spectrumof oxazine (a in FIG. 6B) changes dramatically after the addition ofBu₄NOH (b in FIG. 6B). In particular, the chemical shift of the protonH^(a) increases by 0.15 ppm with the transformation of oxazine into thehemiaminal. The signals of the other aromatic protons (H^(b)—H^(h)),instead, move in the opposite direction. The largest change is observedfor the resonances associated with the proton H^(h), whose chemicalshift decreases by 0.85 ppm. Furthermore, the AB system associated withthe diastereotopic pair of methylene protons H^(l) and H^(m) ismaintained, confirming the presence of a chiral center also in thehemiaminal. In addition, the formation of the hemiaminal is furtherconfirmed by the presence of a peak at an m/z of 390 in the fast atombombardment mass spectrum.

The two chromophoric fragments of OX1 and OX2 are isolatedelectronically. As a result, their absorption spectra (a in FIGS. 7A and8A) resemble the sum of those (b and c in FIGS. 7A and 8A) of modelindolines (5 and 6 in Table 2) and 4-nitroanisole (7 in Table 2). Themost significant difference is a shift to longer wavelengths for theabsorption associated with the 4-nitrophenyl chromophore. This band iscentered at 307 nm in the spectrum of 7 (Table 2), but at 318 and 316 nmin those of OX1 and OX2, respectively.

TABLE 2 Absorption wavelengths (λ_(max)) and molar extinctioncoefficients (ε) of the oxazines OX1 and OX2 and of the model compounds5-8 in MeCN at 25° C.^(a) Compound λ_(max) (nm) ε (mM⁻¹cm⁻¹) OX1 31810.0 ± 0.5  OX2 316 11.0 ± 0.6  5 283 2.2 ± 0.1 6 281 3.9 ± 0.2 7 30711.1 ± 0.6  8 426 32.5 ± 0.9  ^(a)Structures of 5-8 are shown below.λ_(max) and ε of the phenolate 8 were determined by recording theabsorption spectrum of the corresponding phenol in the presence ofBu₄NOH (4 equiv.).

The spectra of OX1 and OX2 (a in FIGS. 7A and 8A) do not show bands inthe wavelength range expected for a 4-nitrophenolate chromophore (e inFIGS. 7B and 8B). Thus, the stationary concentrations of the ring-openedisomers of OX1 and OX2 are below the detection limit in both cases,under these experimental conditions (MeCN, 25° C.). After the additionof Bu₄NOH to solutions of OX1 and OX2, however, a band at about 430 nmappears in the spectra of both species (d in FIGS. 7B and 8B). This bandresembles the absorption (e in FIGS. 7B and 8B) of the model phenolate(Table 2) and can be assigned to the ring-opened products detected by ¹HNMR spectroscopy (see FIGS. 4B and 6B).

Transient absorption spectra, recorded 30 ns after laser excitation, ofaerated MeCN solutions of OX1 and OX2 show bands centered at about 440nm (a in FIGS. 9A and 10A). In both instances, the transient bandsresemble the steady-state ones (b in FIGS. 9A and 10A) and, therefore,can be assigned to ground-state absorptions of the 4-nitrophenolatechromophores. In agreement with this assignment, singlet-oxygenmeasurements for OX2 and control experiments with 7 confirm that thetransient absorptions are not associated with a triplet-triplettransitions. Indeed, the quantum yield of singlet oxygen is less than0.02 and the transient spectrum of 7 does not reveal any detectableabsorption in the nanosecond domain.

The kinetic traces monitored at 440 nm (c in FIGS. 9B and 10B) indicatethat the ring-opened isomers are formed within the excitation pulse(about 6 ns). The corresponding quantum yields can be estimated to be0.03 and 0.1 for OX1 and OX2, respectively. In both instances, theabsorbance decays mono-exponentially to zero (d in FIGS. 9B and 10B)with a first-order rate constant of about 4×10⁷ s⁻¹, as the ring-openedisomers revert thermally to OX1 and OX2.

The interconversion of the two enantiomers of oxazine demands thethermal opening of the oxazine ring with formation of indolium.Nonetheless, the stationary concentration of indolium is negligible andthe steady-state absorption spectrum (a in FIGS. 11A and 12A) revealsonly a band at 308 nm for the p-nitrophenoxy chromophore of OX2. Indeed,this absorption resembles the one observed for p-nitroanisole (b inFIGS. 11A and 12A), under identical experimental conditions. Thecharacteristic band at 429 nm (c in FIGS. 11A and 12A) expected for thep-nitrophenolate component of indolium cannot, instead, be detected (ain FIGS. 11A and 12A). After the addition of Bu₄NOH and continuousirradiation of OX2, however, an intense absorption (d in FIG. 11A) for ap-nitrophenolate chromophore appears in the spectrum. Thus, theexcitation of OX2 encourages the formation of indolium, which is“trapped” in the form of a hemiaminal (FIG. 6A) after attachment of thenucleophilic hydroxide anion to the electrophilic carbon of the indoliumcation. Consistently, the fast atom bombardment mass spectrum recordedat this point shows a peak at a m/z of 390 for the hemiaminal.

Laser flash photolysis measurements confirm the photoinduced opening ofthe oxazine ring with the formation of indolium. Indeed, the transientabsorption spectrum of OX2 (e in FIG. 11B), recorded 30 ns after laserexcitation, shows a band at 440 nm, which can be assigned to ap-nitrophenolate chromophore (c to d in FIG. 11A). Consistent with thisassignment, control experiments with p-nitroanisole exclude a possibleassociation of this transient absorption with the triplet state of thep-nitrophenoxy fragment of oxazine. In fact, no transient absorptionsare observed for p-nitroanisole, under identical experimentalconditions. Furthermore, the quantum yield of singlet oxygen is lessthan 0.02, when the photoinduced conversion of oxazine into indolium isperformed in air-saturated MeCN.

Steady-state absorption spectra (a in FIGS. 12A and 13A) ofpolymethylmethacrylate (PMMA) films doped with OX1 or OX2 areessentially identical to those recorded in MeCN (see a in FIGS. 7A and8A). Once again, the stationary concentrations of the ring-openedisomers are negligible and the absorption bands of their4-nitrophenolate chromophores cannot be detected. In the presence ofBu₄NOH, however, the characteristic absorptions of hemiaminal forms atabout 430 nm can be observed also in PMMA (b in FIGS. 12A and 13A).Similar bands are evident in the transient absorption spectra of OX1 andOX2 (c in FIGS. 12A and 13A) recorded 1 μs after laser excitation. Theseabsorptions can be assigned to the photogenerated ring-opened isomersand are relatively short lived. After 80 μs, for example, the band at430 nm can no longer be observed in the transient spectrum of OX2 (d inFIG. 13A). In addition to this absorption band, a second and moreintense band at 380 nm (c in FIGS. 12-13) is evident in the transientspectra of both compounds. This absorption is relatively long lived andcan still be observed after the complete decay of the band at 430 nm (din FIG. 13A). The corresponding kinetic traces (d in FIG. 12B and e inFIG. 13B) show bi-exponential decay with rate constants of 1×10⁴ and1×10⁵ s⁻¹ for OX1 and of 3×10³ and 5×10⁴ s⁻¹ for OX2. Both trendsparallel the behavior of nitrospiropyrans in polymer matrices. Indeed,the thermal decoloration of nitrospiropyrans also follows bi-exponentialkinetics under these experimental conditions. The aggregation of theirphotogene-rated isomers into long-lived supramolecular assemblies isbelieved to be responsible for this behavior. Presumably, similarprocesses govern the spectral evolution of OX1 and OX2 in PMMA. In anycase, the absorbance associated with the photogenerated species of bothsystems can be modulated reversibly with millisecond switching speeds.Once again, both photochromic switches are remarkably stable and remainessentially unaffected after thousands of excitation cycles. As anexample, FIG. 14 illustrates kinetic traces recorded at 380 nm for OX2before and after 1000 switching cycles. The profiles are virtuallyindistinguishable indicating that the photochromic switch is, indeed,extremely stable.

The evolution of the absorbance for OX1 (d to e in FIG. 12B) and OX2(FIG. 15) indicates that the formation of indolium occurs within thelaser pulse of about 6 ns. The quantum yield for the photoinducedconversion of OX2 into its indolium isomer is about 0.1. A kineticanalysis of the absorbance decay at 440 nm shows that the photogeneratedindolium isomer reverts thermally to OX2 with a first-order rateconstant of (46±1)×10⁶ s⁻¹. Therefore, the starting state is restoredwithin about 50 ns.

Furthermore, the reversible interconversion between OX1 and OX2 andtheir indolium isomers is not accompanied by photodegradation, even inair-saturated solutions. Indeed, the transient absorption andsteady-state spectra recorded before and after more than 3000 excitationcycles, in the presence of molecular oxygen, are virtually identical.Thus, substantially all of the [1,3]oxazine compound is recovered intacteven after thousands of cycles. The remarkable photochemical stabilityof the [1,3]oxazine compounds agrees with their inability to sensitizeefficiently the form ation of singlet oxygen, which is responsible inpart for the degradation of spiropyrans.

The absorbances of the [1,3]oxazine compound and its phenolatederivative (see FIG. 16) can be altered and reset with nanosecondswitching speeds by turning a light source on and off. The short timescales of these processes correspond to an improvement of ten orders ofmagnitude over any of our earlier all-optical processing schemes. Theelimination of the sluggish trans→cis step, limiting the thermalre-isomerization of spiropyrans, has translated into this dramaticdecrease in switching times. In addition, this structural modificationhas conferred remarkable stability on the photoresponsive molecularskeleton. The photoinduced and reversible interconversion between the[1,3]oxazine compound and its phenolate derivative can be achieved withnanosecond switching speeds. Furthermore, the significant changes indipole moment and molecular polarizability accompanying thephotoisomerization can, in principle, be exploited to photoregulate adiversity of material properties with unprecedented switching speeds.Thus, our molecular design for the realization of fast and stablephotochromic compounds can evolve into the development of a new familyof photoresponsive materials.

We have identified an innovative structural design to developphotochromic compounds with improved switching times and fatigueresistance. It is based on the fusion of indoline and benzooxazinefragments into a single molecular skeleton as shown below. The[1,3]oxazine ring in the compound (A) opens upon illumination. Thecleavage of a [C—O] bond responsible for [1,3]ring opening is extremelyfast and produces a phenolate derivative (B) with a switching time ofless than a nanosecond. The photogenerated isomer also reverts thermallyto the starting compound in nano-seconds. Thus, this particularmolecular design offers the opportunity to switch between a colorless(A) and a colored (B) state in nanoseconds by simply turning on and offa visible light source. Furthermore, the process is not accompanied byphotodegradation, even in the presence of molecular oxygen. Ourphotochromic compounds survive thousands of switching cycles withoutdecomposing. In addition, our structural design tolerates a variety ofsubstituents (R¹, R², R³, R⁴, R⁵, or any combination thereof) on theindoline and benzooxazine fragments. These groups can be manipulated toregulate the excitation wavelength of A and the color of B, offeringaccess to a new family of photochromic compounds with unprecedentedswitching speeds and stability.

Materials & Methods

Chemicals were purchased from commercial sources and used as receivedwith the exception of MeCN and CH₂Cl₂, which were distilled over CaH₂.2-Methyl-3,3′-dimethyl-3H-indole (1) was purchased from a commercialsource.

All reactions were monitored by thin layer chromatography, usingaluminum sheets coated with silica (60, F₂₅₄). High-performance liquidchromatography (HPLC) was performed with analytical (columndimensions=4.6×250 mm, flow rate=1.0 mL min⁻¹, injection volume=10 μL,sample concentration=0.1 mM) and semi-preparative (columndimensions=21.4×250 mm, flow rate=10 mL min⁻¹, injection volume=10 mL,sample concentration=0.1 mM). The retention time (RT) and the peakasymmetry (PA) were determined at a wavelength of 254 nm. The averagepurity parameter (APP) was calculated for the peak heart in thewavelength range 215 nm to 700 nm. Melting points (mp) were determinedwith an Electrothermal Mel-Temp apparatus and are uncorrected. Fast atombombardment mass spectra (FABMS) were recorded with a VG Mass Lab Trio-2spectrometer in a 3-nitrobenzyl alcohol matrix.

Nuclear magnetic resonance (NMR) spectra were recorded with a BrukerAvance 400 or a Bruker Avance 500 spectrometer. Steady-state absorptionspectra were recorded with a Varian Cary 100 Bio spectrometer, either inaerated MeCN using quartz cells with a path length of 0.5 cm or inpoly(methyl methacylate) (PMMA) matrices. The polymer films wereprepared by spin-coating aliquots of CH₂Cl₂ solutions of PMMA (160 mgmL⁻¹) and either OX1 or OX2 (8 mg mL⁻¹) with and without Bu₄NOH (7equiv.) on glass plates at 429 rpm for 9 s. The thickness of theresulting films were about 6 μm and were measured with a digitalmicrometer. Transient absorption spectra were recorded with a commerciallaser flash photolysis apparatus either in aerated MeCN, using quartzcells with a path length of 1.0 cm, or in PMMA matrices. The excitationsource was a Nd:YAG laser (355 nm, 6 ns, 8 or 12 mJ). The quantum yield(Φ) for the photoinduced ring opening of OX1 and OX2 was determined witheq. 1, using an optically matched MeCN solution of benzophenone asstandard. The quantum yield (Φ_(BE)) for the intersystem crossing ofbenzophenone is unity, and the molar extinction coefficient (ε_(BE)) forits triplet absorption at 520 nm is 6.5 mM⁻¹ cm⁻¹. The molar extinctioncoefficient of the ring-opened isomers at 440 nM was estimated to beabout 22 mM⁻¹ cm⁻¹. The terms in χ and χ_(BE) are the slopes of thelinear portions of plots of the photoinduced absorbance changes,measured at the end of the pulse, for the ring-opened isomer and thebenzophenone triplet, respectively, against the energy of the laserpulse. Samples were irradiated continuously with a Mineralight UVGL-25lamp (365 nm, 10 min) when necessary. The output power (400 μW cm⁻²) wasdetermined with a Newport 1815-C meter.

2-Phenyl-3,3′-dimethyl-3H-indole (2). A mixture of phenylhydrazine (1.1mL, 11 mmol), i-propylphenylketone (1.5 mL, 10 mmol) andp-toluenesulfonic acid (0.11 g, 0.6 mmol) was heated under reflux for 7h. After cooling down to ambient temperature, the mixture was dilutedwith a saturated aqueous solution of NaHCO₃ (10 mL) and extracted withCHCl₃ (4×10 mL). The organic phase was dried (MgSO₄) and the solvent wasevaporated under reduced pressure. The residue was dissolved in absoluteEtOH (35 mL) and, after the addition of ZnCl₂ (14 g, 0.1 mol), washeated under reflux for 24 h. After cooling down to ambient temperature,the mixture was diluted with a saturated aqueous solution of NaHCO₃ (40mL) and extracted with Et₂O (3×10 mL). The organic phase was dried(MgSO₄) and the solvent was evaporated under reduced pressure. Theresidue was purified by column chromatography [SiO₂: CH₂Cl₂/heptane(4:1)] to afford the product (1.86 g, 84%) as an orange liquid. FABMS:m/z=222 [M]⁺; ¹H-NMR (500 MHz, CDCl₃): δ 1.55 (6H, s), 7.30 (1H, d, 8Hz), 7.34-7.40 (2H, m), 7.47-7.53 (3H, m), 7.72 (1H, d, 8 Hz), 8.14-8.20(2H, m); ¹³C-NMR (100 MHz, CDCl₃): δ 24.7, 53.5, 120.7, 120.9, 126.0,127.7, 128.3, 128.4, 128.5, 130.6, 132.9, 147.4, 183.3.

2-Nitro-5a,6,6-trimethyl-5a,6-dihydro-12H-indolo[2,1-b][1,3]benzooxazine(OX1). A solution of 1 (291 μL, 1.8 mmol) and2-chloromethyl-4-nitrophenol (162 mg, 0.9 mmol) in MeCN (5 mL) wasstirred for 50 min at ambient temperature under N₂. Then, the mixturewas stored in a refrigerator for 12 h. The resulting precipitate wasfiltered and dissolved in H₂O (40 mL). After the addition of aqueous KOH(0.05 M, 5 mL), the solution was extracted with Et₂O (3×20 mL). Theorganic layer was dried (MgSO₄) and filtered, and the solvent wasdistilled off under reduced pressure to give OX1 (94 mg, 0.3 mmol) as awhite solid. The mother liquor of the initial filtration wasconcentrated under reduced pressure, and the residue was purified bycolumn chromatography [SiO₂/CH₂Cl₂→CH₂Cl₂/MeCO₂Et (10:1)] to afford anadditional amount of OX1 (60 mg, 0.2 mmol). The overall yield of OX1 was58%. HPLC [analytical, MeCN/H₂O (80:20)]: RT=4.5 min, PA=1.6,APP=236.7±1.0 nm; mp=180° C.; FABMS: m/z=311 [M+H]⁺; ¹H-NMR (500 MHz,CDCl₃): δ 1.17 (3H, s), 1.52 (3H, s), 1.57 (1H, d, 11 Hz), 4.63 (1H, d,11 Hz), 6.73 (1H, d, 8 Hz), 6.86-6.94 (2H, m), 7.16-7.19 (3H, s), 4.60(2H, m), 6.56 (1H, d, 8 Hz), 6.69 (1H, d, 9 Hz), 6.82 (1H, d, t, 8 Hz),7.07 (1H, t, 9 Hz), 7.11 (1H, d, 8 Hz), 7.92 (1H, dd, 3 and 9 Hz), 8.06(1H, d, 3 Hz); ¹³C-NMR (75 MHz, CDCl₃): δ 16.6, 18.9, 26.0, 40.0, 48.0,102.8, 108.4, 118.2, 118.8, 120.5, 122.3, 123.3, 124.0, 127.6, 138.0,140.4, 146.6, 159.1.

2-Nitro-5a-phenyl-6,6-dimethyl-5a,6-dihydro-12H-indolo[2,1-b][1,3]benzooxazine(OX2). A solution of 2 (700 mg, 3.2 mmol) and2-chloromethyl-4-nitrophenol (709 mg, 3.8 mmol) in MeCN (30 mL) washeated under reflux for 48 h. After cooling down to ambient temperature,the solvent was evaporated under reduced pressure and the residue wasdissolved in CH₂Cl₂ (30 mL). The resulting solution was washed withaqueous KOH (0.2 M, 15 mL) and H₂O (15 mL). The organic phase wasconcentrated under reduced pressure and the residue was purified bycolumn chromatography [SiO₂: hexane→CH₂Cl₂/hexane (1:1 v/v)] to give OX2(680 mg, 58%) as a white solid. HPLC [analytical, MeCN/H₂O (95:5)]:RT=3.6 min, PA=2.1, APP=261.0±1.0 nm; mp=176° C.; FABMS: m/z=372 [M]⁺;¹H-NMR (400 MHz, CDCl₃): δ 0.89 (3H, s), 1.60 (3H, s), 4.53 (1H, d, 11Hz), 4.63 (1H, d, 11 Hz), 6.73 (1H, d, 8 Hz), 6.86-6.94 (2H, m),7.16-7.19 (2H, m), 7.38-7.42 (3H, m), 7.54-7.65 (2H, m), 7.92-7.94 (2H,m); ¹³C-NMR (100 MHz, CDCl₃): δ 18.6, 27.9, 41.0, 49.9, 105.5, 109.2,118.3, 120.3, 121.0, 122.6, 123.3, 123.9, 127.9, 128.2, 128.8, 129.1,136.1, 137.9, 141.4, 147.0, 159.3.

Steady-Sate Absorption Spectroscopy. The absorption spectra wererecorded either in aerated MeCN, using quartz cells with a path lengthof 0.5 cm, or in polymethylmethacrylate (PMMA) matrices. The polymerfilms were prepared by spin-coating aliquots of CH₂Cl₂ solutions of PMMA(160 mg mL⁻¹) and either OX1 or OX2 (8 mg mL⁻¹) with and without Bu₄NOH(7 equiv.) on glass plates at 420 rpm for 9 s. The thicknesses of theresulting films was about 6 μm and was measured with a digitalmicrometer.

Laser Flash Photolysis. Solutions of oxazine (0.1 mM, 22° C.±2° C.) inMeCN were illuminated with the third harmonic of a Nd—YAG ContinuumSurelite II-10 laser (355 nm, 6 ns, 8 mJ), using quartz cells with apath length of 1.0 cm. The excited solutions were analyzed with aLuzchem Research mLFP-111 apparatus with an orthogonal pump/probeconfiguration. The probe source was a ceramic xenon lamp coupled toquartz fiber-optical cables. The laser pulse and the mLFP-111 systemwere synchronized by a Tektronix TDS 3032 digitizer, operating inpre-trigger mode. The signals from a compact Hamamatsu photomultiplierwere initially captured by the digitizer and then transferred to apersonal computer, controlled by Luzchem Research software operating inthe National Instruments LabView 5.1 environment. The energy of thelaser pulse was measured at each shot with a SPHD25 Scientechpyroelectric meter.

Determination of the Quantum Yield for the Transformation of Oxazineinto Indolium. The quantum yield (Φ_(IN)) for the photoinducedconversion of oxazine into indolium was determined with equation (1),using an optically-matched MeCN solution of benzophenone as standard.The quantum yield (Φ_(BE)) for the intersystem crossing of benzophenoneis unity and the molar extinction coefficient (ε_(BE)) for its tripletabsorption at 520 nm is 6.5 mM⁻¹ cm⁻¹. The molar extinction coefficient(ε_(IN)) of indolium at 440 nm was estimated to be about 22 mM⁻¹ cm⁻¹from the absorption spectrum of potassium p-nitrophenolate. The termsχ_(IN) and χ_(BE) in equation (1) are the slopes of the linear portionsof plots of the photoinduced absorbance changes, measured at the end ofthe pulse for indolium and the benzophenone triplet respectively,against the energy of the laser pulse.

$\begin{matrix}{\Phi_{IN} = \frac{\chi_{IN}ɛ_{BE}\Phi_{BE}}{\chi_{BE}ɛ_{IN}}} & (1)\end{matrix}$

Determination of the Quantum Yield of Singlet Oxygen. The evolution ofsinglet oxygen (¹Δ_(g)) in the course of the photoinduced transformationof oxazine into indolium was monitored by luminescence measurements inair-saturated MeCN. Upon laser excitation of oxazine, the emission ofsinglet oxygen at 1.27 μμm was probed orthogonally to the exciting beamwith a pre-amplified (low impedance) Ge-photodiode (Hamamatsu EI-P, 300ns resolution) maintained at −196° C. and coupled to a long-pass siliconfilter (>1.1 μm) and an interference filter (1.27 μm). The temporalprofile of the luminescence was fitted to a single-exponential decayfunction with the exclusion of the initial portion of the plot, which isaffected by the scattered excitation. The luminescence at initial timewas extrapolated from the curve fitting. The quantum yield (Φ_(Δ1)) ofsinglet oxygen was determined with equation (2), using anoptically-matched and air-saturated MeCN solution of benzophenone asstandard. The quantum yield (Φ_(Δ2)) of singlet oxygen formed uponexcitation of benzophenone is 0.37. The terms χ₁ and χ₂ in equation (2)are the slopes of the linear portions of plots of the singlet-oxygenluminescence, determined at initial time upon excitation of oxazone andbenzophenone respectively, against the energy of the laser pulse.

$\begin{matrix}{\Phi_{\Delta\; 1} = \frac{\chi_{1}\Phi_{\Delta\; 2}}{\chi_{2}}} & (2)\end{matrix}$

Some of the above results are disclosed in Tomasulo et al. (Org. Lett.7:1109-1112, 2005), Tomasulo et al. (J. Org. Chem. 70: 8180-8189, 2005),and associated supporting information; all of which are alsoincorporated by reference herein in their entirety.

Six additional compounds have been synthesized. Compounds 9 and 13-14were synthesized in accordance with the above. Compounds 10-12 weresynthesized by displacing a methyl at R⁴ by aldehyde condensation. Theconjugated systems at R¹ and R⁴ are expected to shift absorbance(λ_(max)) to longer wavelengths.

In stating a numerical range, it should be understood that all valueswithin the range are also described (e.g., one to ten also includesevery integer value between one and ten as well as all intermediateranges such as two to ten, one to five, and three to eight).

All modifications and substitutions that come within the meaning of theclaims and the range of their legal equivalents are to be embracedwithin their scope. A claim which recites “comprising” allows theinclusion of other elements to be within the scope of the claim; theinvention is also described by such claims reciting the transitionalphrases “consisting essentially of” (i.e., allowing the inclusion ofother elements to be within the scope of the claim if they do notmaterially affect operation of the invention) or “consisting of” (i.e.,allowing only the elements listed in the claim other than impurities orinconsequential activities which are ordinarily associated with theinvention) instead of the “comprising” term. Any of these threetransitions can be used to claim the invention.

It should be understood that an element described in this specificationshould not be construed as a limitation of the claimed invention unlessit is explicitly recited in the claims. Thus, the granted claims are thebasis for determining the scope of legal protection instead of alimitation from the specification which is read into the claims. Incontradistinction, the prior art is explicitly excluded from theinvention to the extent of specific embodiments that would anticipatethe claimed invention or destroy novelty.

Moreover, no particular relationship between or among limitations of aclaim is intended unless such relationship is explicitly recited in theclaim (e.g., the arrangement of components in a product claim or orderof steps in a method claim is not a limitation of the claim unlessexplicitly stated to be so). All possible combinations and permutationsof individual elements disclosed herein are considered to be aspects ofthe invention. Similarly, generalizations of the invention's descriptionare considered to be part of the invention.

From the foregoing, it would be apparent to a person of skill in thisart that the invention can be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments should be considered only as illustrative, not restrictive,because the scope of the legal protection provided for the inventionwill be indicated by the appended claims rather than by thisspecification.

1. A method of optically switching a photochromic compound or materialincorporating said compound, wherein said compound is a substituted[1,3]oxazine as shown in Formula I

wherein said R¹ is selected from the group consisting of hydrogen,hydroxyl, C1-C4 alkyls, substituted C1-C4 alkyls, C5-C6 cycloalkyls,substituted C5-C6 cycloalkyls, C5-C6 aryls, substituted C5-C6 aryls,C5-C6 heterocycles, and substituted C5-C6 heterocycles; said R² isselected from the group consisting of hydrogen, hydroxyl, C1-C4 alkyls,substituted C1-C4 alkyls, C5-C6 cycloalkyls, substituted C5-C6cycloalkyls, C5-C6 aryls, substituted C5-C6 aryls, C5-C6 heterocycles,and substituted C5-C6 heterocycles; said R³ is selected from the groupconsisting of hydrogen, hydroxyl, C1-C4 alkyls, substituted C1-C4alkyls, C5-C6 cycloalkyls, substituted C5-C6 cycloalkyls, C5-C6 aryls,substituted C5-C6 aryls, C5-C6 heterocycles, and substituted C5-C6heterocycles; said R⁴ is selected from the group consisting of hydrogen,hydroxyl, C1-C4 alkyls, substituted C1-C4 alkyls, C5-C6 cycloalkyls,substituted C5-C6 cycloalkyls, C5-C6 aryls, substituted C5-C6 aryls,C5-C6 heterocycles, and substituted C5-C6 heterocycles; and said R⁵ is anitrogen-containing group or any other electron withdrawing substituent;said method comprising irradiating said compound or materialincorporating said compound with light to switch optically from thecompound to its phenolate derivative, wherein light induces cleavage ofa [C—O] bond in the compound's oxazine ring to produce the phenolatederivative by ring cleavage.
 2. The method according to claim 1, whereinsaid compound switches to its phenolate derivative in 250 ns or less. 3.The method according to claim 1, wherein said switch results inabsorbance of visible light to shift from colorless to colored when thecompound isomerizes to its phenolate derivative.
 4. The method accordingto claim 1, wherein said switch results in the maximum absorbancewavelength to shift between the compound and its phenolate derivative bya difference of at least 50 nm.
 5. The method according to claim 1,wherein said switch results in the maximum absorbance wavelength toshift between the compound and its phenolate derivative by a differenceof at least 200 nm.
 6. The method according to claim 1, wherein saidlight has a wavelength from 200 nm to 800 nm.
 7. The method according toclaim 1, wherein said light has a wavelength from 800 nm to 1300 nm. 8.The method according to claim 1 further comprising not irradiating saidphenolate derivative to switch back by thermal re-isomerization.
 9. Themethod according to claim 1 further comprising trapping said compound inhemiaminal form by using a nucleophile to compete with ring closing. 10.The method according to claim 1, wherein isomerization between saidcompound and its phenolate derivative is possible over greater than 1000excitation cycles.